A 3rd generation CT scanner has a finite measured field-of-view (hereinafter “MFOV”) defined by the shortest distance from the isocenter to rays extending from the x-ray source to the edge detectors. The MFOV of a typical CT scanner is 50 cm, which is sufficient for radiology. However, when CT is combined with another modality, such as radiation therapy or PET, the patient is positioned to place the target area (typically a tumor) closer to isocenter. As a result, the limbs of the patient may be positioned outside of the MFOV of the CT subsystem. Consequently, extension of the reconstructed field-of-view (hereinafter the “RFOV”) up to 60-70 cm may be needed to capture the entire cross-section of the region of the patient that is of interest.
If a part of the patient is located outside of the MFOV, the measured projections are truncated at one or both edges (the edges being defined by the two edges of the MFOV where the outmost rays extending from source to the edge detectors intersect the MFOV). Images reconstructed from truncated projections have characteristic bright artifacts which corrupt the image inside and outside of the MFOV. In order to avoid truncation artifacts and recover the anatomy outside of the MFOV, the truncated projections need to be extended beyond the MFOV.
One approach to extending truncated projections is based on parallel (rebinned) projections, where the total amount of attenuation in a view does not change with view index. If at least one non-truncated projection is available, then the total attenuation of each view is known, so that the total attenuation in the extension for each truncated view may be computed.
If a view is truncated on both edges, the missing attenuation needs to be distributed between the left extension and the right extension. If the position of the center-of-mass (COM) of the patient is known, the missing attenuation in the doubly truncated projection is distributed such that the center of mass of the extended parallel projection agrees with the COM.
At least one known extension technique is optimized for anatomical objects. However, truncation may also be caused by non-anatomical interfering objects, such as (a) reinforced edges of the pallet of the patient table; (b) restraining equipment; (c) equipment for intravenous injections. Interfering objects generally increase the total amount of truncation and reduce the quality of the extension. Thus a method and system for extending truncated projections in the presence of interfering objects is needed.
One technique for extending the RFOV in fan projection space is described in U.S. Patent Application Publication 2007/0076933 (Starman et al.). The advantages of the algorithm described in the Starman et al. application are improved accuracy of CT numbers in the extended region(s), better stability under various truncation scenarios, and better extension when a large portion of anatomy is truncated. However, at least two disadvantages of the algorithm described in the reference are longer development time and slower speed.